The present invention relates to a capacitive sensing array device comprising an array of sensing electrodes which are covered by a layer of electrically insulating material defining a sensing surface spaced from the sensing electrodes. The invention is concerned particularly, but not exclusively, with a fingerprint sensing device in which the array of sensing electrodes are used to sense capacitively the pattern of ridges and valleys of a person's fingerprint when placed over the sensing surface.
Examples of capacitive sensing array devices of the above kind, particularly fingerprint sensing devices, are described in U.S. Pat. No. 5,325,442 and British Patent Application No. 9608747.3. In these example devices a row and column matrix array of sense elements is provided on an insulating substrate using thin film technology. Each sense element comprises a metal sensing electrode and at least one switching device, in the form of a TFT (thin film transistor), connected thereto. The switching devices of the sense elements are connected to a peripheral drive circuit via sets of row and column address conductors. The array of sensing electrodes is covered by a layer of deposited insulating material comprising silicon nitride or polyimide material whose surface remote from the sensing electrodes defines a substantially planar sensing surface over which a person's finger is placed, the sensing electrodes together with their overlying dielectric material and individual fingerprint portions, i.e. ridges and valleys, constituting capacitors. The capacitances of these individual capacitors are sensed by means of the drive circuit applying charges to the sense electrodes, via their associated address conductors and switching devices at the sense elements, on a row at a time basis and measuring the supplied charge. The capacitance of the individual capacitors is dependent on the spacing between a sensing electrode and an overlying fingerprint portion, as determined by the thickness of the dielectric layer and the presence of a fingerprint ridge or valley, and through measuring this charge for each sense element in the array by scanning each row of sense elements in turn an electronic image or representation of the three dimensional form of the fingerprint surface provided by the variation in sensed capacitances produced over the array by the fingerprint ridge pattern is obtained.